Check out filterdirect or Aquasafe Canada. Both have good units. Most of the 6 stage units are really only 5 stage. The 6th stage is a carbon filter for taste. Not needed for the fish but good for drinking.

I would think you could fill the tank and then put a separate valve in before the 6th stage to fill a container for the tank when needed.

That is the exact unit I bought for my 90gal. I would recommend it. It is pretty easy to install. I have about 15 gal jugs laying around that I fill up to keep water on hand. My experience is that this unit takes about 2 hours to fill the tank and the tank will hold just less than 3 gals at a time. You must plan ahead for water changes, but for the price and convenience I would say buy it.

Just a note for drinking RO/DI water, dont use it as your only source of water, as it is so soft (like no minerals at all) it has a high tendency to rid your body of nutrients.

Also, if you do drink ro/di water make it fresh water, dont let it sit out for too long as it absorbs CO2 quite readily and will turn the water acidic, which will slowly build up in your body, I have a report from some doctor that I found....